docs: mention deprecated attribute with argument (#9218)

doc/docs.md

@@ -4021,6 +4021,10 @@ function/struct/enum declaration and applies only to the following declaration.
 fn old_function() {
 }
 
+// It can also display a custom deprecation message
+[deprecated: 'use new_function() instead']
+fn legacy_function() {}
+
 // This function's calls will be inlined.
 [inline]
 fn inlined_function() {